A phone rings in the dark. A man reaches from his camp bed, switches on a light and groans when he sees the time is ten minutes to three in the morning. He grunts a few words down the phone and nudges awake another man sleeping nearby.

The two walk out the hut the few yards to where their helicopters are parked.

They climb in and work through pre-flight checks. The engine whines and the drooping rotor blades sweep around, slowly at first but within seconds straightening and becoming a blur. The two machines pull themselves into the sky and head off at low level, the whump-whump noise echoing off mountains edging the wide valley.

The call had been expected, that is why they were sleeping by their machines. They too checked weather forecasts and closely followed the thermometer’s sinking mercury.

They weren’t flying passengers nor carrying urgent cargo, and they weren’t travelling anywhere. Instead they would fly a few miles to a farm and hover at low altitude over the fields until they needed to refuel or the danger was over. The fields were growing grapevines and the downwind from the helicopter’s rotor blades would keep the expected frost at bay.

Frost in spring can wipe out a year’s crop by killing new growth, flowers and berries. Air movement prevents frost from forming and one reason many traditional vineyards are planted up on slopes is because of that. Cold air descends down hillsides leaving vines clear from frost that forms at the bottom of the slope. Wine is able to be grown in otherwise inhospitable places such as the Finger Lakes in northern New York State and in Niagara on the banks of Lake Ontario in Canada just because of the proximity of large areas of water causing air movements down the slopes of the lake banks.

Vineyards planted on the flat and susceptible to frost have various methods of countering frost. One is smudge-pots. These are small containers of oil set along the rows, placed every few vines that are set alight, the heat they emit just raising the temperature by enough. In the past some places burned rubbish and old car tyres but foul smoke brings complaints from neighbours and nearby towns.

Others have fixed large propeller blades on high poles among the vines to move the air. These are the most obvious sign of frost control and you may well have seen them in vineyards and maybe thought they were wind-powered generators, but they’re just the opposite.

Others, where frost doesn’t come every year such as in the Marlborough wine region of New Zealand, find it more economical to hire helicopters when needed rather than lose their crop.